Some MIDI controllers have the ability to "detach" their keyboards
from the internal sound-generating hardware. In this state, playing
the keyboard produces MIDI output, but no sound. Incoming MIDI data
is still sent to the sound producing section of the device. Thus you
can, for example, play a slave device from the keyboard while another
device such as a sequencer plays the master controller.

MIDI provides a message for turning this feature on and off, called
"Local Control". When Local Control is on, a keyboard's notes
are sent to its own synth as well as generate MIDI output. This
is the normal state of affairs. When Local Control is off, keyboard
data is sent out to MIDI only.

Some MIDI devices send out an "active sensing" message every
so often when they are not transmitting any other MIDI data. This
can confuse or slow down some other MIDI devices. Turning the active
sensing filter on tells KeyMaster to block any incoming
active sensing messages from being sent out.